1. Field of the Invention
This invention relates to detection of the presence or absence of defects, such as photoresist or electron beam resist stains, on a non-visible surface of a semiconductor wafer during wafer processing.
2. Background of the Technical Art
When a semiconductor wafer is processed to produce one or more integrated circuits (ICs), a semiconductor substrate, such as a silicon wafer, is provided. The integrated circuit is then constructed on the semiconductor substrate. Certain parts of the IC may require a selective process, such as chemical vapor deposition, chemical etching or ion implantation as part of the wafer processing. In these selective processes, photoresist (.PHI.R) material or electron beam resist (EBR) material is laid down (and later removed) to selectively expose certain parts of a substrate or wafer face for the particular processing step. The processing step may be chemical vapor deposition, chemical etching, ion implantation or a similar processing step. The exposed or "front" wafer surface is observed directly after application of the .PHI.R or EBR material to that surface. However, a non-visible or "back" surface of the wafer rests upon or is adjacent to a support surface and normally cannot be observed. An unwanted portion of the .PHI.R or EBR material used on a wafer front face occasionally appears on the wafer back surface. This unwanted material can produce chemical stains that degrade either the performance or the appearance of the wafer or of an IC fabricated on the wafer, or that interfere with proper operation of subsequently applied equipment.
During certain semiconductor wafer processing steps, a support surface such as a chillplate, maintained at a temperature that is less than the wafer temperature, is placed contiguous to a back surface of the wafer to reduce the temperature of the wafer. When this temperature reduction occurs, certain unwanted semiconductor processing materials that are present on the back surface of the wafer may change phase and produce a chemical stain or other defect on the back surface and/or on the chillplate surface that is contiguous to the back surface.
What is needed is an technique that allows such stains or similar defects on a back surface of a semiconductor wafer to be detected as soon as possible after such stains or defects first appear in the processing of a wafer. Preferably, this technique would allow the location and physical extent of the defect(s) to be determined visually and would allow the severity of the defect(s) to be evaluated without manually removing the wafer from its support surface or chillplate surface.